The present invention relates to a gear change for an automobile vehicle and in particular, but not exclusively, to a gear change for an industrial vehicle.
Gear changes comprising several trains in cascade are known, for instance from Italian Patent 01251251 in the name of the applicants; industrial vehicles are, for instance, generally provided with a gear change comprising a main train defining a certain number of forward ratios and at least one reverse ratio and an output stage with two ratios (direct take-up and reduction) calculated to define, in combination with the main train, two separate series or ranks of ratios.
The output stage is generally formed by an epicyclic gear train with straight teeth, comprising a sun gear rigid with the output shaft of the main train, or primary shaft, a plurality of planet gears borne by a train carrier rigid with an output shaft of the gear change and an outer crown moving in an axial direction between a first operating position in which it is angularly rigid with the gear box (in order to provide a reduction ratio) and a second operating position in which it is angularly rigid with the train carrier (in order to provide for direct take-up). The crown is born, moreover, exclusively by the planet gears, with respect to which it may undergo small displacements in the radial direction allowing for self-centring in the various operating positions and avoiding damaging forcing of the engaged teeth.
These epicyclic gear trains have the drawback that they generate a relatively high level of noise during operation which is propagated via the gear box and the bodywork and can be heard in an irritating manner in the vehicle's passenger space.
In order to remedy this drawback it could be envisaged to increase the sound absorbing capacity of the gear box. Sound absorbing materials could be used for this purpose, although these are generally problematic because of their relatively high specific weight, or the thickness of the sheet metal walls could be increased; these solutions are obviously not compatible with the current trend in the automobile engineering field to reduce the weight of automobile vehicles.
As an alternative, it could be envisaged to provide the gears of the epicyclic gear train with helical teeth which are less noisy than gears with straight teeth. This would, however, raise the problem of supporting the axial forces transmitted, in operation, to the crown which tend to displace it from the operating position in which it is located. This drawback could obviously not be resolved by supporting the crown by collar bearings if it is wished to retain the possibility of self-centring of the crown with respect to the planet gears.